Rankine cycle power generation systems, such as steam power plants, have long been in use as a source of electric power. A typical steam power plant includes a boiler which heats a pressurized working fluid (typically water) to a high temperature, and high pressure. The high temperature high pressure steam is then fed to a turbine where it is expanded to a lower pressure and where it is also reduced in temperature. The turbine outputs power from the power plant. Thereafter, the steam is typically condensed back to a liquid and then pumped to a higher pressure before being returned to the boiler.
The amount of electric power outputted by the power plant is a function of the amount of heat energy put into the water at the boiler and a function of the overall efficiency of the power plant. Numerous techniques are utilized to enhance the efficiency of the power plant. In many power plants multiple turbines are provided with progressively lower inlet pressures and inlet temperatures so that all of the available energy in the working fluid can be extracted. In many such power plants reheaters are provided, typically in the form of additional boilers, which reheat the working fluid between the multiple turbines.
Another efficiency enhancing technique involves increasing a temperature of the steam, particularly at the discharge from the boiler and the inlet to the turbine. When a temperature difference between the inlet of the highest pressure turbine and the discharge of the lowest pressure turbine is increased, the efficiency of the power generation system is increased. However, the constraints of different materials available for use within the boiler and within the turbine restrict the practical temperatures which can be achieved at the inlets to the turbine, hence limiting maximum attainable efficiency.
While high temperatures (above 1050° F.) do present some challenges in turbine design and operation, boiler maximum temperature limitations have been the primary impediment to increasing inlet temperatures for turbines within steam power plants. Hence, while steam turbines having inlet temperatures higher than 1050° F. (for example) might be relatively easily designed and manufactured, the relative difficulty in providing boilers that can provide steam at temperatures above 1050° F. have made the development of such higher temperature turbines unimportant.
Recently new methods for generating high temperature high pressure working fluids for Rankine cycle power generation have been introduced, making possible higher temperature steam, and potentially correspondingly higher efficiency within Rankine cycle power generation systems. Specifically, U.S. Pat. Nos. 5,473,899; 5,590,528; 5,680,764; 5,709,077; 5,715,673; 5,956,937; 5,970,702; 6,170,264; 6,206,684; 6,247,316; 6,389,814; and 6,523,349, incorporated herein by reference, describe in detail a gas generator which combusts a fuel, typically a hydrocarbon fuel, but optionally hydrogen, syngas from coal or other sources, etc. with oxygen to produce a working fluid of steam and carbon dioxide. As the oxidizer is oxygen rather than air, temperatures of up to 3000° F. are attainable, with temperatures of over 1500° F. readily obtained in such gas generators.
Disadvantageously, existing steam turbines of appropriate inlet pressures have been developed for lower temperatures than 1500° F. Hence, power generation systems utilizing such gas generators require additional water or other diluent to be added to the working fluid to drop the temperature from over 1500° F. down to approximately 1050° F., so that no damage is done to the turbines. This dilution of the working fluid and reduction in temperature decreases the overall efficiency of the power generation system.
With such power generation systems, providing reheaters between the high pressure turbine and lower pressure turbines increases the efficiency of such power generation systems somewhat. However, further increases in efficiency would still further enhance the attractiveness of such power generation systems. As such oxyfuel combustion based power generation systems produce products of combustion of substantially only steam and carbon dioxide, such power generation systems hold tremendous promise in eliminating the air pollution typically generated by combustion based power generation systems. According, a need exists for ways to enhance the efficiency of such power generation systems without requiring turbines having inlet temperatures greater than those already exhibited by existing steam turbines, such as approximately 1050° F.